The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for current regulation. Merely by way of example, the invention has been applied to power conversion systems. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) are widely used for lighting applications. Oftentimes, the currents flowing through LEDs need to be approximately constant. The output-current precision of LEDs is usually used for determining the constant-current properties of a LED lighting system.
FIG. 1 is a simplified diagram showing a conventional power conversation system for LED lighting. The power conversion system 100 includes a controller 102, resistors 104, 124, 126 and 132, capacitors 106, 120 and 134, a diode 108, a transformer 110 including a primary winding 112, a secondary winding 114 and an auxiliary winding 116, a power switch 128, a current sensing resistor 130, and a rectifying diode 118. The controller 102 includes terminals 138, 140, 142, 144, 146 and 148. For example, the power switch 128 is a bipolar junction transistor. In another example, the power switch 128 is a MOS transistor.
An alternate-current (AC) input voltage 152 is applied to the system 100. A bulk voltage 150 (e.g., a rectified voltage no smaller than 0 V) associated with the AC input voltage 152 is received by the resistor 104. The capacitor 106 is charged in response to the bulk voltage 150, and a voltage 154 is provided to the controller 102 at the terminal 138 (e.g., terminal VCC). If the voltage 154 is larger than a predetermined threshold voltage (e.g., a under-voltage lock-out threshold) in magnitude, the controller 102 begins to operate normally, and outputs a driving signal 156 through the terminal 142 (e.g., terminal GATE). For example, the driving signal 156 is a pulse-width-modulation (PWM) signal with a switching frequency and a duty cycle. The switch 128 is closed (e.g., being turned on) or open (e.g., being turned off) in response to the driving signal 156 so that the output current 158 is regulated to be approximately constant.
The auxiliary winding 116 charges the capacitor 106 through the diode 108 when the switch 128 is closed (e.g., being turned on) in response to the driving signal 156 so that the controller 102 can operate normally. A feedback signal 160 is provided to the controller 102 through the terminal 140 (e.g., terminal FB) in order to detect the ending of a demagnetization process of the secondary winding 118 for charging or discharging the capacitor 134 using an internal error amplifier in the controller 102. The resistor 130 is used for detecting a primary current 162 flowing through the primary winding 112, and a current-sensing signal 164 is provided to the controller 102 through the terminal 144 (e.g., terminal CS) to be processed during each switching cycle. Peak magnitudes of the current-sensing signal 164 are sampled and provided to the internal error amplifier. The capacitor 120 is used to keep an output voltage 168 stable.
FIG. 2 is a simplified conventional diagram showing the controller 102 as part of the system 100. The controller 102 includes an oscillator 202, an under-voltage lock-out (UVLO) component 204, a modulation component 206, a logic controller 208, a driving component 210, a demagnetization detector 212, an error amplifier 216, and a current-sensing component 214.
As shown in FIG. 2, the UVLO component 204 detects the signal 154 and outputs a signal 218. If the signal 154 is larger than a predetermined threshold in magnitude, the controller 102 begins to operate normally. If the signal 154 is smaller than the predetermined threshold in magnitude, the controller 102 is turned off. The error amplifier 216 receives a signal 220 from the current-sensing component 214 and a reference signal 222 and outputs an amplified signal 224 to the modulation component 206. The modulation component 206 also receives a signal 228 from the oscillator 202 and outputs a modulation signal 226 which is a PWM signal. For example, the signal 228 is a ramping signal and increases, linearly or non-linearly, to a peak magnitude during each switching period. In another example, the modulation signal 226 has a fixed switching frequency and the duty cycle of the signal 226 is determined based on a comparison between the signal 224 and the signal 228. The logic controller 208 processes the modulation signal 226 and outputs a control signal 230 to the driving component 210 which generates the signal 156 to turn on or off the switch 128. The demagnetization detector 212 detects the feedback signal 160 and outputs a signal 232 for determining the beginning and the end of the demagnetization process of the secondary winding 114.
FIG. 3 is a simplified conventional diagram showing the current-sensing component 214 and the error amplifier 216 as parts of the controller 102. The current-sensing component 214 includes a switch 302 and a capacitor 304. The error amplifier 216 includes switches 306 and 308, an operational amplifier 310.
As shown in FIG. 3, the current-sensing component 214 samples the current-sensing signal 164 and the error amplifier 216 amplifies the difference between the signal 220 and the reference signal 222. Specifically, the switch 302 is closed (e.g., being turned on) or open (e.g., being turned off) in response to a signal 314 in order to sample peak magnitudes of the current-sensing signal 164 in different switching periods. If the switch 302 is closed (e.g., being turned on) in response to the signal 314 and the switch 306 is open (e.g., being turned off) in response to the signal 232 from the demagnetization detector 212, the capacitor 304 is charged and the signal 220 increases in magnitude. If the switch 306 is closed (e.g., being turned on) in response to the signal 232, the switch 308 is open (e.g., being turned off) in response to a signal 312 and the difference between the signal 220 and the reference signal 222 is amplified by the amplifier 310. For example, during the demagnetization process of the secondary winding 114, the signal 232 is at a logic high level. The switch 306 remains closed (e.g., being turned on) and the switch 308 remains open (e.g., being turned off). The amplifier 310, together with the capacitor 134, performs integration associated with the signal 220.
Under stable normal operations, an average output current is determined, according to the following equation, without taking into account any error current:
                                          I            o                    _                =                              1            2                    ×          N          ×                                    V              ref_ea                                      R              cs                                                          (                  Equation          ⁢                                          ⁢          1                )            where N represents a turns ratio between the primary winding 112 and the secondary winding 114, Vref_ea represents the reference signal 222 and Rcs represents the resistance of the resistor 130.
But the system 100 has problems in regulating the output current to be approximately constant. Hence it is highly desirable to improve the techniques of regulating output currents of power conversion systems.